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I was having chronic problems trying to print a retraction test with my Ender 3. I had recently replaced the stock extruder with a Micro Swiss all metal hot end. Being a bit "ham-fisted", I managed to break the thermistor soon after that. While changing that, I learned a bit (maybe not enough) about thermistortable11.h, calibrating thermistor profiles and configuring Marlin. Long story short, I got myself deeper and deeper into trouble until I could not print anything without clogging up the heat break by the time the printer started layer 2.

I found a useful thread on the same issue in another group and the mystery was solved.
Since I don't see a similar discussion here, I thought I would post the link: https://www.thingiverse.com/groups/i3/forums/general/topic:6463

Basically, one tradeoff of using the all-metal hot end is that retraction distances more than 4mm are likely to pull softened filament up into the cool end, the extruder may then "jam" the soft filament in the heat break, when it pushes back down.
I have still to investigate how much stringing this limitation is going to leave me dealing with, but at least restricting my retraction distance to 1.5mm in Cura has resolved that clogging problem.
BTW - The Cura 4.3.0 end G code for Ender 3 also retracts the filament at the end of a job. For some reason, two successive commands each retract by 2mm, for a total of 4mm. Hoping I don't regret it, I have removed one of those retractions, thinking the worst case will be when a print job ends with a retraction command, followed by the end Gcode retraction command(s).